The Electoral Commission have released a study by NOP into how many people in London aren’t actually registered to vote. NOP found 20% of people in London between 18 and 25 weren’t registered and 27% of people between 25 and 34 weren’t. In comparison, only 5% of over 65s and 8% of 55-64s are unregistered.
It looks as though these are mostly likely to be disproportionately votes that would otherwise go to Ken Livingstone. Older people are the most pro-Boris group and registration also tends to be lower amongst people in rented accomodation and people from ethnic minorities – a mainstay of Ken’s support.
Some polls – most notably Ipsos MORI’s last poll for Unison – did ask whether people were registered to vote and excluded those who weren’t, but this doesn’t actually go that far to addressing the question because it assumes people who aren’t on the register know they aren’t. The NOP study cross-references the answers they got with the electoral register, and found that 16% of respondents who thought they were registered weren’t actually on it.